The scope of an optical communication system has been expanded from a trunk line system to a subscriber system, a local area network system, a data link system etc. in accordance with the progress of an optical communication technology. In order to comply with the highly advanced technology of an optical communication system, optical devices must be of high performance and function.
An optoelectronic integrated circuit is one of key devices which is a nucleus in such an optical communication system. Therefore, research and development of the optoelectronic integrated circuit are focused not only to the basical advantages of circuit integration such as low-cost, compact size, high reliability and non-adjusting maintenance, but also to the improvement of performances such as high speed and high sensitivity, and further to the realization of a device with high and new function which supports a future optical system such as an optical interconnection and an optical switching.
In order to realize such an optoelectronic integrated circuit, a study on composite materials of GaAs system and InP system from which a optimum combination of materials is obtained has been conducted as described on page 215 of "Appl. Phys. Lett. vol. 49. 1986" by M. Razeghi wherein InP system material is for an optical device and GaAs system material is for an electronic device The optoelectronic integrated circuit described therein is of a monolithic integrated circuit including an GaInAs optical device which is grown on a semi-insulating GaAs substrate by the strain heteroepitaxy based on the low pressure MOVPE, and an electronic device which is a GaAs MESFET.
According to the conventional optoelectronic integrated circuit, however, there are disadvantages that a yield in the fabrication thereof is lowered, and that fine and minute process for providing a gate of the MESFET is difficult to be performed, because the difference of horizontal levels is resulted between the optical device and the electronic device for the reason why InP system optical device layer is of a thickness of more than several .mu.m.